[skottie] Stroke dash support

AE supports dashing all strokes.  Dashes are specified as an arbitrary
number of intervals (alternating dash/gap) plus a start offset.

All values can be animated independently (but of course!).

  - implement a SkSG dash effect (based on SkDashPathEffect)
  - expand the shape builder logic to allow local geometry adjustments
    (kind of a bummer that dashing is a stroke/paint property as opposed
    to a geometry effect in AE)

Change-Id: Ic9ff35f2f9a552a3c26f9e1596ce58ad81f7ced5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274550
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
6 files changed